Traditionally, brittle ballistic armor materials have two primary failure mechanisms: (1) a failure wave ahead of the projectile creates a path of comminuted material that provides little resistance to the projectile, resulting in penetration; and/or (2) spalling on the back of the armor removes significant material ahead of the projectile, allowing penetration. [1] Both of these failure mechanisms are directly associated with the shockwave created by a ballistic impact. In two dimensions, the shockwave in a material can be visualized by dropping a pebble into a pond; the rings of outward propagating waves are the shockwaves in the water. To carry the analogy further, hurricane-produced waves crashing into a sea wall are higher energy shockwaves. The shockwave travels many times faster than the projectile and as stated above, can be very destructive to a material.
In the case of the failure wave, shockwaves are induced within the material as a result of rapid compression of the material's atoms and/or ions (in the remainder of this paragraph, the collective “atoms” refers to both atoms and ions) from the ballistic impact. The shockwave is in fact compressed atoms that travel, as do the waves in the pond. Just behind the shockwave front the atoms are forced much closer together than in their equilibrium state and are then violently repelled by interatomic forces further from the shockwave front. If the repelled atoms overcome their mutually attractive forces, the material fractures from these tensile forces. If the repelled atoms do not overcome their mutually attractive forces, they are violently dragged back together, pass the equilibrium point and end up oscillating like a spring. These oscillations are arbitrarily quantized and are called phonons. There are two general types. Optical phonons have very high energy and vibrate at frequencies where the atoms cannot vibrate in a coherent form. As the vibrations dampen, the optical phonons loose energy i.e. vibration speed, and are termed acoustic phonons. Although acoustic phonons do not necessarily produce sound nor vibrate at acoustic frequencies audible to the human ear, they do propagate similarly to acoustic waves in air (hence the name), as alternating bands of compressed and uncompressed atoms. The propagating acoustic phonon waves are termed lattice waves. As the shockwave moves through the material, it loses energy by generating lattice waves. Technically, it is the higher energy lattice waves that cause material fracture, which is why the failure wave and resulting fracture forms behind the shockwave front. If the energy density of the shockwave is below a specific threshold, lower energy lattice waves do not cause fracture unless several low energy lattice waves combine through constructive interference. In summary, there is a fundamental energy associated with the excitation of each of the phonons created that is generated as a result of the increased atomic motion and energy. The higher the energy of the phonon, the shorter its lifetime before it decays into multiple lower energy phonons. As these high energy phonons decay, they increase the intensity of existing lower energy phonons and once their atomic vibrational amplitude exceeds the strength of the material, it begins to fracture, giving rise to the failure wave. [2] [3]
Spalling on the back of armor result from a large high to low impedance mismatch between the back of the armor and what is behind it; generally air which has very low impedance. The behavior of shockwaves within a medium is largely controlled by the acoustic impedance of that medium. The acoustic impedance of a material is the product of the materials density and speed of sound (the mass flux). When a shockwave encounters an interface formed by two materials having a large acoustic impedance mismatch a large portion of the shockwave is reflected back into the material in which the shockwave was initially traveling while the rest is transmitted into the second material. The reflected shockwave is compressive in the case of a low to high impedance interface and tensile in the case of a high to low impedance interface e.g. the back of the armor where is meets the air. The amount of reflection and transmission is directly related to the difference in the impedance between the two materials. [4]A larger difference in the impedance of the two materials results in a larger amount of shockwave reflection. As the shockwave reflects off the armor/air interface, it creates a tensile wave. If the intensity of reflected tensile wave is higher than the tensile strength of the material, failure occurs in the form of spalling. For imperfect interfaces, there is an additional shockwave response known as scattering which means that the reflected or transmitted shockwave is deviated from its straight trajectory. Scattering can be caused by lower impedance imperfections within a material or at an interface that has a non-planar geometry such as roughness. [5]
Traditional ballistic protection seeks to defeat projectiles using materials that possess high strength, hardness, and fracture toughness arranged with low impedance mismatch at material interfaces to allow the shockwave to travel unimpeded as far as possible. [1] [6] [7] [8] This allows the penetrator a longer residence time in unspalled material causing the material to fail under compression instead of tension. In the case of transparent armor, this typically involves three to six layers of materials having thicknesses in the range of millimeters to centimeters, bonded to one another using various epoxies and other adhesives. [6] [9] [10] Previous approaches to improved ballistic performance against a given projectile have been to either find a stronger, harder, tougher material; or to add more of an existing armor material in between the projectile and the target. [11] [12] [13] This often results in heavy and bulky armor systems or systems decreasing mobility of the personnel and the mobility of their vehicles (this term includes land, air, and sea vehicles) and which leads to lower survivability. Additionally, there are many platforms (this term includes vehicles and fixed installations such as buildings) that have no ballistic protection because of design constraints. The result is that most armor systems have to make a tradeoff between performance, weight, and size.
There are newer armor systems where some secondary thought is given to shockwave reflection, but it is limited to uncontrolled scattering of the shockwave and resulting random generation of lattice waves. [4] [14] [15] [16] [17] The problem with these uncontrolled approaches is that the randomly scattered waves recombine through constructive interference, are not guided away from the penetrator, and cause fracture to the material ahead of the penetrator.
A need exists for techniques to control shockwave behavior within armor materials in order to minimize or eliminate the above two main failure mechanisms in traditional armor while minimizing armor weight and bulk.